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A fossil normally preserves only a portion of the deceased organism, usually that portion that was partially mineralized during life, such as the bones and teeth of vertebrates, or the chitinous or calcareous exoskeletons of invertebrates.
If this hole is later filled with other minerals, it is a cast.
The degree to which the remains are decayed when covered determines the later details of the fossil.
Some fossils consist only of skeletal remains or teeth; other fossils contain traces of skin, feathers or even soft tissues. In some cases the original remains of the organism completely dissolve or are otherwise destroyed.
In some cases mineral replacement of the original shell occurs so gradually and at such fine scales that microstructural features are preserved despite the total loss of original material.
A shell is said to be recrystallized when the original skeletal compounds are still present but in a different crystal form, as from aragonite to calcite.